Xylenes are valuable industrial chemicals. Sources of xylenes include catalytic reformate, pyrolysis gasoline, toluene disproportionation; C.sub.7 -C.sub.9 aromatic transalkylation, and the like. For example, catalytic reforming hydrocarbon feeds such as naphtha using conventional aromatization catalysts produces a reformate which is richer in the content of C.sub.6 -C.sub.10 aromatics than the feeds. Of these aromatics, significant quantities of C.sub.8 aromatics are produced which comprise a mixture of ethyl benzene, and mixed ortho-, meta- and para-xylene isomers. Typically, the product from the catalytic reformer (reformate) is fed to an aromatic extraction plant where the aromatics, e.g., C.sub.6, C.sub.7 and C.sub.8 aromatics, are separated from the paraffins and other non-aromatic products present in the reformate. The C.sub.8 aromatic fraction may then be separated from the lower boiling C.sub.6 and C.sub.7 aromatics by distillation.
The C.sub.8 aromatic fraction normally contains a mixture of ethyl benzene and the ortho-, para-, and meta-xylene isomers. The three xylene isomers are usually present in near thermodynamic equilibrium amounts, e.g., generally 52-53 wt. % meta-xylene, 23-24 wt. % para-xylene and 23.5 to 24.5 wt. % ortho-xylene. Of the xylene isomers, meta-xylene is typically the least desired product. Because para-xylene is of particular value as a chemical intermediate in a number of applications, it may be desirable to separate the para-xylene from the other isomers using conventional techniques such as crystallization, or by adsorption/desorption on zeolites. After such separation, the residual C.sub.8 aromatic fraction contains non-equilibrium quantities of ethylbenzene and the mixed ortho- and meta-xylene isomers and is lean with respect to para-xylene content.
The para-xylene lean residual product may be further upgraded by subjecting it to isomerization conditions wherein at least a portion of the ethylbenzene is converted to other products such as diethylbenzene or benzene and ethane and a portion of the ortho- and meta-xylenes are isomerized to produce a mixture which once again approximates the equilibrium concentration of the ortho-, meta-, and para-xylene isomers. Typically such isomerization conditions comprise contacting the non-equilibrium C.sub.8 aromatic feed with a suitable isomerization catalyst in a suitable reactor at temperatures above about 600.degree. F. and preferably at pressures sufficient to maintain the reaction in the vapor phase.
A commercially viable xylene isomerization process must exhibit high xylene isomerization activity and, also, must produce the desired product without a significant loss of xylenes. The loss of xylene is a result of undesired side-reactions, involving hydrogenation of the aromatic ring, hydrogenolysis, demethylation, and particularly disproportionation and transalkylation.
Another factor of importance in a xylene isomerization process is the effect that ethylbenzene has on the entire isomerization and xylene recovery loop. When ethylbenzene is present in appreciable quantities in the feed to the isomerization process, it will accumulate in the loop unless it is excluded from the feed or converted by some reaction in the loop to products which are separable from xylenes. Ethylbenzene can be separated from the xylenes by "superfractionation", but this procedure is very expensive. A more desirable method of eliminating the ethylbenzene is through a conversion reaction taking place simultaneously with the isomerization reaction of the xylenes. One method of converting ethylbenzene is to isomerize the ethylbenzene to xylenes. It is often desirable that the ethylbenzene conversion reaction be a deethylation reaction producing benzene and ethane rather than a disproportionation reaction to benzene and diethylbenzene. The deethylation reaction preserves more xylenes and produces a high quality reaction product.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Numerous processes have been proposed for the isomerization of xylene feeds using zeolite catalysts. For instance, U.S. Pat. No. 4,312,790 involves a xylene isomerization process using an alumina bound zeolite catalyst. U.S. Pat. No. 4,939,110 involves a xylene isomerization process using a zeolite catalyst such as a ZSM-5 which is bound by a binder material such as alumina, silica, or clay.
Synthetic zeolites are normally prepared by the crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound.
The zeolite powder is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used for xylene isomerization, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in xylene isomerization. Furthermore, when the bound zeolite is used in xylene isomerization, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products.